FIG. 13 is a perspective view of an operator's cab frame constituting a structural support member of an operator's cab and FIG. 14 is a cross section along line S--S in FIG. 13. As illustrated in FIGS. 13 and 14, an operator's cab in the prior art is mainly constituted of a base plate 50A provided as the bottom surface, a rear panel 51 as the rear surface, which are provided standing on the base plate 50A, and side panels 51A, each constituted of an outer panel 52 and an inner panel 53 as the left side surface or the right side surface, which are provided standing on the base plate 50A, bonded with each other one on top of the other. To the left and right at the front end, the left and right at the center and the left and right at the rear end of the operator's seat, the outer panels 52 and inner panels 53 form hollow first pillars 54A and 54B, second pillars 55A and 55B and third pillars 56A and 56B respectively, extending in the vertical direction. A roof 57 is provided atop the pillars 54A and 54B through 56A and 56B. Various parts are provided as illustrated in FIG. 15 in the rear of the inside of the operator's cab having such a frame structure. In FIG. 15, supports 59 are provided standing on the upper surface of the baseplate 50B behind the operator's seat 58, with a plate 60 provided across between the upper ends of the supports 59. A storage tray 61 is mounted at the upper surface of the plate 60 and an air conditioning system 62 is mounted at the upper surface of the base plate 50B below the plate 60.